The majority of combustible fuel used in the world today is derived from crude oil. There are several limitations to using crude oil as a fuel source. Crude oil is in limited supply; it includes aromatic compounds believed to cause cancer, and contains sulfur and nitrogen-containing compounds that can adversely affect the environment, for example, by producing acid rain.
Combustible liquid fuels can also be prepared from natural gas. This involves converting the natural gas, which is mostly methane, to synthesis gas, or syngas, which is a mixture of carbon monoxide and hydrogen. An advantage of using fuels prepared from syngas is that they do not contain nitrogen and sulfur and generally do not contain aromatic compounds. Accordingly, they have minimal health and environmental impact.
Fischer-Tropsch chemistry is typically used to convert the syngas to a product stream that includes combustible fuel, among other products. A limitation associated with Fischer-Tropsch chemistry is that it tends to produce a broad spectrum of products, ranging from methane to wax. Product slates for syngas conversion over Fischer-Tropsch catalysts (Fe, Co and Ru) are controlled by polymerization kinetics with fairly constant chain growth probabilities, which fix the possible product distributions. Heavy products with a relatively high wax content are produced when chain growth probabilities are high. Methane is produced with high selectivity when chain growth probabilities are low.
Methane can be recirculated to ultimately yield combustible liquid fuel. Wax can be processed, for example, by hydrocracking and/or hydrotreating followed by oligomerization, to yield combustible liquid fuel. However, it would be advantageous to have new methods for providing a product stream from a Fischer-Tropsch process that has a higher proportion of combustible liquid fuel with less methane to recirculate and less wax to process.
One method used in the past to minimize methane production has been to incorporate olefins in the Fischer-Tropsch reaction. Work in the early 1930's used a roughly 1:1 ratio of hydrogen/carbon monoxide, and added olefins to the reaction mixture (Smith et al., J.A.C.S., 52:3221 (1930). This tended to provide oxygenated material, which is not preferred. U.S. Pat. No. 4,754,092 to Iglesia et al. discloses incorporating olefins into a Fischer-Tropsch reaction, but does not specify the type of chain growth probabilities for the reaction, and discloses using a wide range of hydrogen/carbon monoxide ratios such that it would be difficult to predict whether the product would be oxygenated, olefinic, or saturated.
It would be advantageous to provide methods for improving product yields in Fischer-Tropsch reactions, while minimizing methane and oxygenate production. The present invention provides such methods.